1. Field of the Invention
This invention relates to a process for the synthesis of the porous crystalline material ZSM-12, to ZSM-12 produced by said process, and to use of the resultant ZSM-12 as a catalyst for organic compound, e.g., hydrocarbon compound, conversion.
More particularly, this invention relates to a process for synthesizing aluminosilicate ZSM-12 having a low silica to alumina molar ratio and substantially free of impurity phases.
2. Discussion of the Prior Art
ZSM-12 and its conventional preparation in the presence of a tetramethylammonium or tetraethylammonium directing agent are taught by U.S. Pat. No. 3,832,449, the entire disclosure of which is incorporated herein by reference. ZSM-12 has a distinctive X-ray diffraction pattern which distinguishes it from other known crystalline materials.
U.S. Pat. No. 4,391,785 discloses a method for the synthesis of ZSM-12 from a reaction mixture comprising, as a directing agent, a compound selected from the group consisting of a dimethyl pyridinium halide and a dimethyl pyrrolidinium halide.
U.S. Pat. Nos. 4,452,769 and 4,537,758 disclose methods for synthesizing ZSM-12 from a reaction mixture containing methyltriethylammonium ions as the directing agent.
Other organic directing agents that have been used to synthesize ZSM-12 include bis (dimethylpiperidinium) trimethylene ions (see U.S. Pat. No. 4,539,193), benzyltriethylammonium ions (see U.S. Pat. No. 4,552,738), dimethyldiethylammonium ions (see U.S. Pat. No. 4, 552,739), benzyltrimethylammonium ions (see U.S. Pat. No. 4,585,637), bis (N-methylpyridyl) ethylinium ions (see U.S. Pat. No. 4,5852,746) and bis (methylpyrrolidinium)-diquat-n, where n=4, 5 or 6 (see U.S. Pat. No. 5,192,521).
Hexamethyleneimine has been used as a directing agent to synthesize a variety of different crystalline structures, including MCM-22 (U.S. Pat. No. 4,954,325), MCM-35 (U.S. Pat. No. 4,981,663) and ZSM-12 (U.S. Pat. No. 5,021,141). In particular, the examples of U.S. Pat. No. 5,021,141 disclose the use of hexamethyleneimine to synthesize ZSM-12 with silica/alumina mole ratios varying between 62 and 852. ZSM-12 synthesized using hexamethyleneimine as the directing agent tends to crystallize as large hexagonal platelets having edge dimensions of about 2000 nm.
Existing methods for the synthesis of ZSM-12 suffer from the problem that they tend to produce impurity phases, such as ZSM-5, especially when at product silica/alumina molar ratios less than 100. According to the present invention, it has now been found that a novel directing agent, the N,N-dimethylhexamethyleneimine cation, can be used to synthesize ZSM-12 at silica/alumina molar ratios below 50 with little or no coproduction of impurity phases. In addition, ZSM-12 synthesized using the N,N-dimethylhexamethyleneimine cation as the directing agent tends to crystallize as small crystals with dimensions of about 50 nm.
It is to be appreciated that, although ZSM-12 is normally synthesized as an aluminosilicate, the framework aluminum can be partially or completely replaced by other trivalent elements, such as boron, iron and/or gallium, and the framework silicon can be partially or completely replaced by other tetravalent elements such as germanium.
According to the invention, there is provided a process for synthesizing the porous, crystalline material ZSM-12 which comprises the steps of:
(i) preparing a mixture capable of forming said material, said mixture comprising sources of alkali or alkaline earth metal (M), an oxide of trivalent element (X), an oxide of tetravalent element (Y), hydroxyl (OHxe2x88x92) ions, water and N,N-dimethylhexamethyleneimine ions (R), wherein said mixture has a composition, in terms of mole ratios, within the following ranges:
(ii) maintaining said mixture under sufficient conditions until crystals of said material are formed; and
(iii) recovering said crystalline material from step (ii).
Preferably, said reaction mixture has a composition in terms of mole ratios within the following ranges:
Preferably, M is sodium.
Preferably, said mixture also contains a source of tetramethylammonium ions.